Method of actinically imaging

ABSTRACT

A lithographic printing plate or other coated imageable substrate is imaged by heating an area of the coating with an infrared laser and reacting the coating in the heated area with ultraviolet or visible radiation. The coating can be either positive working or negative working. The modulated radiation may either be the ultraviolet/visible radiation or the infrared radiation and the radiation spots are superimposed or the ultraviolet/visible spot may closely trail the infrared spot. The imaging time is reduced since the reaction rate is increased at the elevated temperature.

This application is a continuation-in-part application of U.S. patentapplication Ser. No. 09/482,483 filed Jan. 12, 2000 now abandoned.

BACKGROUND OF THE INVENTION

The present invention relates to the imaging of a plate or board havinga coating imageable by ultraviolet or visible radiation and morespecifically to actinic imaging with simultaneous heating to improve theimaging process. The invention is particularly directed to the imagingof lithographic printing plates.

One type of imageable lithographic printing plate or other imageableplate is a negative-working, actinic plate which has a resin coatingnormally soluble in a developer and which is rendered insoluble whenexposed to radiation, usually in the ultraviolet range. The plate isimaged by exposing the coating to the radiation in those areascorresponding to the image to be printed with those areas becominginsoluble and ink receptive. Another type of imageable lithographicprinting plate is a positive-working plate which is also actinicallyimaged. This type of plate has a resin coating normally insoluble in adeveloper which is rendered soluble when exposed to the radiation.

Actinic imaging, whether it be by ultraviolet radiation or by visibleradiation, can be accomplished by one of two techniques. One techniqueis to expose the printing plate through a film negative. The otherapproach is to serially scan the plate with small image spots or areas.This latter approach can be accomplished by digital laser imaging or bya method referred to as digital screen imaging which will be explainedlater. Although digital screen imaging is not digital imaging in thestrictest sense of that term as will be explained, the term “digital”will be used herein to encompass both the digital laser imaging and thedigital screen imaging which both involve imaging by serially scanningthe plate with small areas or spots of the imaging radiation.

SUMMARY OF THE INVENTION

An object of the present invention is to provide an improved actinicimaging method for either negative-working or positive-working plates orboards such as lithographic printing plates or printed circuit boards.The invention involves the imaging of the plate to cause a reaction inthe coating in the exposed areas by actinic imaging using eitherultraviolet or visible radiation in combination with infrared radiation.The reaction may either solubilize or insolubilize the coating dependingon whether the plate is positive- or negative-working. The infraredradiation operates to increase the localized temperature of the coatingto a level at which the rate of the actinic imaging reaction isincreased. Either the ultraviolet/visible radiation or the infraredradiation can be image modulated. The relative areas covered by theultraviolet/visible radiation and by the infrared radiation can bevaried such that the areas are superimposed or that the area of theultraviolet/visible radiation closely trails the infrared area.

DESCRIPTION OF THE PREFERRED EMBODIMENT

The invention relates to the imaging of plates or boards with imageablecoatings such as lithographic printing plates and circuit boards. Theinvention will be described with particular reference to lithographicprinting plates but it is to be understood that the description alsoapplies to such other products. Lithographic printing plates withcoatings which are imageable by ultraviolet or visible radiation whereinthe radiation causes an insolubilizing reaction in the coating fornegative-working plates and a solubilizing reaction for positive-workingplates are well known. Typically, the insolubilizing reaction fornegative-working plates is a crosslinking or photopolymerizationreaction but other chemical changes may also insolubilize the coatingand are within the scope of the invention. The insolubilized imagedareas become the ink-receptive plate areas and the non-imaged areas areremoved with a typical developer solution. The coatings for such platesare well known in the art and are typically a diazo resin havingreactive sites which are capable of being chemically altered by theradiation. A suitable diazo resin is the condensation product of3-methoxy-4 diazo-diphenylamine and paraformaldehyde. Other suitablediazo compounds are described in a variety of prior patents includingU.S. Pat. Nos. 5,998,095; 4,956,261; 3,406,159; 3,277,074; 3,311,605;3,163,633 and 3,050,502. For positive-working plates, the coatings arenormally insoluble in the developer solution and are solubilized in theareas exposed to the imaging radiation. Such coatings are well known andusually comprise a diazide, such as diazonaphthoquinone derivatives,mixed with or reacted with a phenolic resin. For a specific disclosureof diazonaphthoquinone derivatives, see U.S. Pat. No. 5,858,626. Themechanism of such coatings is described in the article, “The MolecularMechanism of Novolak Resins” by Arnost Reiser appearing in the Journalof Imaging Science and Technology, Volume 42, No. 1, January/February1998 on pages 15 to 22.

Lithographic printing plates may be imaged by digital laser imaging andby digital screen imaging. The invention will first be described withrespect to digital laser imaging and the digital screen imaging will bedescribed later. Also, the invention will be described with respect tonegative-working plates with coatings which are insolubilized by theradiation but it is to be recognized that the invention applies equallyto positive-working plates with coatings which are solubilized by theradiation. Furthermore, the invention is applicable to imaging radiationwith a wavelength shorter than infrared. For convenience purposes,ultraviolet radiation or devices will be referred to but it is to beunderstood that the imaging radiation of the invention includes visibleas well as ultraviolet radiation. For example, blue or violet diodelasers or a double frequency YAG laser at 532 nm can be used. Like mostchemical reactions, the rate at which the solubility conversion takesplace for the actinic digital imaging of a lithographic plate isdependent on the temperature. The rate of reaction is slower at ambientconditions than at elevated temperatures. By elevating the temperatureat which the imaging is done, the speed of the imaging process can beincreased. It is problematic to increase the temperature of the entireplate by elevating the temperature of the imaging platen or drum. Thethermal expansion of the components of the imager and the dynamics ofmanaging the heat flow to the plate in a timely and uniform manner makethis approach impractical.

According to the present invention, the temperature of the resin coatingis increased to enhance the reaction but it is only locally andmomentarily heated and heated generally simultaneously with the imaging.The invention involves spot heating the coating with an infrared laserand subjecting the coating while it is heated to the ultravioletradiation to react the coating at the elevated temperature. The coatingis not reacted or otherwise imaged by the infrared radiation but theinfrared radiation interacts with the coating and/or substrate merely toheat the coating. Either the ultraviolet radiation or the infraredradiation can be image modulated as discussed below. If desired, aninfrared absorbing dye may be incorporated into the coating tofacilitate the infrared absorption and heating. These dyes are wellknown in the art and include materials such as squarylium, croconate,cyanine, phthalocyanine, merocyanine, chalcogenopyryloarylidene,orzindollizine, puinoid, indolizine, pyrylium, thizine, azulenium andxanthene dyes. The infrared heating of the coating can also be effectedby using a substrate having infrared absorption characteristics. Forexample, an anodized aluminum substrate which has been rotary brushgrained with calcined alumina so as to embed the graining particles tocover a portion, perhaps 10%, of the surface of the substrate willeffect the rate of heat absorption by the substrate and the conductionof heat away from the spot by the aluminum. The power and intensity ofthe infrared laser as well as the dye in the coating and the nature ofthe substrate can be selected to instantaneously increase the coatingtemperature to enhance the coating reaction by increasing the reactionkinetics. The effect of the coating temperature on the reaction kineticsis progressive, i.e. the higher the temperature, the faster thereaction. Therefore, any temperature increase will have some effect butthe preferred coating temperature range is 250 to 400° F. With thehigher coating temperature, much less power or time is required totrigger the reaction. Specifically, with respect to ultravioletradiation, imaging can be accomplished with much less powerful gaslasers or with a diode or semiconductor ultraviolet laser in the powerrange of one watt or less. Therefore, the lasers are much lessexpensive, require less power input, do not require water cooling andare easier to modulate. The ultraviolet radiation is usually in therange of 340 to 390 nm.

In the practice of the invention, either the infrared radiation or theultraviolet radiation may be image modulated. With modulated ultravioletradiation, the unmodulated infrared radiation heats an area of thecoating which is simultaneously imaged with superimposed modulatedultraviolet radiation. The spot of modulated ultraviolet radiation canbe smaller than or the same size as the spot of infrared radiation.Alternately, the modulated spot of ultraviolet radiation can closelytrail the unmodulated spot of infrared radiation with the criteria beingthat the coating is still hot at the time it is subjected to theultraviolet radiation. With modulated infrared radiation, an area of thecoating is exposed to a relatively low level of unmodulated ultravioletradiation and that spot is simultaneously exposed to the superimposedmodulated infrared radiation. The infrared spot is either the same sizeor smaller than the ultraviolet spot. The level of ultraviolet radiationis low enough that it will not significantly react the coating atambient temperature during the short exposure time. It is only becauseof the simultaneous heating that the ultraviolet radiation is sufficientto react the coating in the heated image pattern formed by the infraredradiation.

In order to locally heat the coating and simultaneously image, the spotson the plate from the infrared laser and the digital imager must belocated such that the imaging is effected while the coating is hot. Thisusually requires that the spots be superimposed although the ultravioletspot may closely trail the infrared spot as noted earlier. Although thespots can be the same size, in order to facilitate superimposition andtaking into consideration the ability to aim and focus the lasers, onespot is preferably larger than the other with the small spot beingentirely within the bounds of the larger spot. The small spot is themodulated radiation whether that be ultraviolet or infrared. Becausethis ultraviolet imaging is a progressive process (rather than athreshold process) in which exposure of the coating to moderate levelsof the ultraviolet radiation could produce some imaging, the ultravioletspot is usually the modulated imaging radiation and is the smaller ofthe two spots with the ultraviolet spot being fully contained within thebounds of the infrared spot. Merely as an example, the large infraredspot may have a diameter of 50 microns while the small digital imagespot has a diameter of 10 microns. If the infrared radiation is themodulated radiation, the infrared spot will be the smaller and the levelof the ultraviolet radiation is kept quite low so that any coatingreaction caused solely by the unmodulated ultraviolet radiation isminimal and insufficient to produce an effective image. Although thisdescription of the invention only contemplates a single pair of infraredand ultraviolet lasers, there can simultaneously be multiple infraredand ultraviolet lasers.

Another method of imaging within the scope of the present invention isby a digital screen imaging system such as marketed by basysPrint Corp.This system uses a radiation source which is usually an ultravioletlight source that passes through a condenser system onto a digitalscreen. This screen has a grid with hundreds of thousands of dots thatcan each be individually electronically controlled. This produces adigital image on the screen that is projected onto the printing plate.It can be seen that it is the screen image that is digital whereas theimage on the plate is a projected image from that digital screen.However, as previously stated, this imaging technique is encompassedwith the scope of the digital imaging of this invention. Because of thesize of the digital screen, only a segment of the plate is imaged in asingle exposure. The segments range in size from 0.3 cm² to 2.5 cm² andcan be projected at the rate of about 10 segments per second. Theexposure head is moved or scanned over the plate to fit the segmentstogether and produce the entire image on the plate. With this type ofdigital imager, one or more infrared lasers are positioned off to theside and aimed at an angle to focus on the area beneath the digitalscreen.

The imaging of the invention may be practiced on any conventionalimaging equipment such as flat bed imagers, internal drum imagers andexternal drum imagers. In order to establish the principal andeffectiveness of the present invention and the benefits achieved whenimaging a coating at an elevated temperature, conventional plates withultraviolet imageable coatings were heated to bring the coated plates upto a selected temperature. The heated plates were then imaged withultraviolet light. The results were compared to unheated plates and toplates which had been heated but cooled prior to imaging.

COMPARATIVE EXAMPLE 1

An Anocoil Waterworks plate, commercially available from AnocoilCorporation of Rockville, Conn., was imaged at 250 mJ/cm² using aTheimer Copymat 64-CP exposure unit having a 2500 watt MuHi spectrumbulb. The exposure negative contained a 21 step Stouffer step wedge.This step wedge is essentially a series of steps with increasing opticaldensity. Each step represents an increase in optical density of 50% morethan the preceding step. The plate was processed through an AnocoilPlate Processor filled with Anocoil S Developer. The step wedge readingon the plate after imaging was a 6.

COMPARATIVE EXAMPLE 2

A plate was heated to a temperature of 90° C. The plate was subsequentlyallowed to cool for five minutes to allow it to return to ambienttemperature conditions. The plate was imaged and processed as inComparative Example 1. The step wedge on the plate was a 6.

EXAMPLE 3

A plate was heated to a temperature of 90° C. and imaged on the Theimerexposure unit while still at elevated temperature. The plate wasprocessed as in Comparative Example 1. The step wedge reading was a 7.This indicates an increase in the imaging speed of approximately 50%over that of Comparative Example 1.

EXAMPLE 4

A plate was made in the manner of Example 3 except that the temperatureto which the plate was heated was 180° C. In this case the step wedgereading was 7.5. This indicates an increase in imaging speed ofapproximately 75% relative to that of Comparative Example 1.

The preceding examples demonstrate the advantage of imaging a platewhile the plate is maintained at elevated temperatures during theultraviolet exposure. Comparative Example 2 demonstrates that themechanism of the present invention differs from other prior art where apreheat is used to effect a change in one component of the coating whichthen renders it imageable. Comparative Example 2 shows that aftercooling the plate has exactly the same imaging characteristics as theunheated plate. There was no conversion of any coating component as aresult of the heating. The present invention relies on the concurrentinfrared heating and imaging exposure processes to effect the selectiveimaging. This establishes the dynamic relationship between the heatingand imaging as practiced with the present invention.

What is claimed is:
 1. A method of imaging a plate having a coatingwhich is actinically imageable by radiation of a selected wavelengthwhich is shorter than the wavelength of infrared radiation and which isin the range of ultraviolet and visible radiation comprising: a.focusing a source of infrared radiation onto a first area on said plateto heat said coating in said first area; b. simultaneously imaging asecond area of said heated coating within the bounds of said first areawith image producing radiation of said selected wavelength; and c.repeating steps a and b for successive areas of said plate.
 2. A methodas recited in claim 1 wherein said coating contains an infraredabsorbing dye.
 3. A method as recited in claim 1 wherein said step ofsimultaneously imaging comprises creating said image producing radiationof said selected wavelength with a modulated laser.
 4. A method asrecited in claim 3 wherein said plate is positive-working and saidcoating is solubilized by said imaging radiation.
 5. A method as recitedin claim 4 wherein said radiation of said selected wavelength isultraviolet radiation and said modulated laser is an ultraviolet laser.6. A method as recited in claim 1 wherein said step of simultaneouslyimaging comprises creating said image producing radiation of saidselected wavelength with a digital screen.
 7. A method as recited inclaim 6 wherein said plate is negative-working and said coating isinsolubilized by said imaging radiation.
 8. A method as recited in claim7 wherein said radiation of said selected wavelength is ultravioletradiation and said step of creating said image producing radiation witha digital screen comprises passing ultraviolet radiation through saiddigital screen onto said coating.
 9. A method as recited in claim 1wherein said plate is a lithographic printing plate.
 10. A method asrecited in claim 1 wherein said plate is a printed circuit board.
 11. Amethod of imaging an ultraviolet sensitive substrate plate having aresin coating capable of being insolubilized by ultraviolet radiationcomprising: a. focusing an unmodulated source of infrared energy onto afirst area on said plate to heat said coating in said first area; and b.simultaneously focusing a source of ultraviolet energy through adigitally generated screen to produce an ultraviolet image within asecond area on said plate, said second area being smaller than saidfirst area and within the bounds of said first area whereby said heatedcoating is insolubilized by said ultraviolet image within said secondarea.
 12. A method as recited in claim 11 wherein said resin coatingcontains an infrared absorbing dye.
 13. A method as recited in claim 11wherein said plate is a lithographic printing plate.
 14. A method asrecited in claim 11 wherein said plate is a printed circuit board.
 15. Amethod of imaging a lithographic printing plate having an imageableresin coating which is insolubilized by ultraviolet radiation comprisingthe steps of: a. scanning said plate with an unmodulated infrared laserbeam focused to a first spot on said plate of a first size to heat saidcoating at said first spot; and b. simultaneously scanning said platewith an ultraviolet image focused to a second spot on said plate of asecond size smaller than said first size and within the bounds of saidfirst spot whereby said heated coating within said first spot isinsolubilized by said ultraviolet image within said second spot.
 16. Amethod as recited in claim 15 wherein said ultraviolet image is createdby a modulated laser beam.
 17. A method as recited in claim 15 whereinsaid ultraviolet image is created by a digital screen.
 18. A method ofimaging a lithographic printing plate having a resin coating which isactinically imageable by radiation of a selected wavelength which is inthe range of ultraviolet and visible radiation and which is shorter thanthe wavelength of infrared radiation comprising: a. focusing anunmodulated infrared laser beam onto a first spot on said plate to heatsaid resin coating at said first spot; and b. simultaneously focusing animage modulated source of energy of said selected wavelength onto asecond spot on said plate, said second spot being smaller than saidfirst spot and within the bounds of said first spot whereby said heatedcoating is actinically imaged within said second spot.
 19. A method asrecited in claim 18 wherein said actinic imaging comprisesinsolubilizing said heated coating.
 20. A method as recited in claim 18wherein said actinic imaging comprises solubilizing said heated coating.21. A method of imaging a plate having a coating which is actinicallyimageable by radiation of a selected wavelength which is shorter thanthe wavelength of infrared radiation and which is in the range ofultraviolet and visible radiation comprising: a. focusing a spot ofinfrared radiation onto an area on said plate to heat said coating insaid area; b. focusing a spot of radiation of said selected wavelengthonto said heated coating in said area; c. image modulating one of saidsources of radiation thereby imaging said coating in said heated area;d. repeating steps a, b and c for successive areas of said plate.
 22. Amethod as recited in claim 21 wherein said radiation of said selectedwavelength is modulated.
 23. A method as recited in claim 22 whereinsaid spot of radiation of said selected wavelength and said spot ofinfrared radiation are simultaneously focused onto said area.
 24. Amethod as recited in claim 23 wherein said spot of radiation of saidselected wavelength and said spot of infrared radiation are the samesize.
 25. A method as recited in claim 23 wherein said spot of radiationof said selected wavelength is smaller than said spot of infraredradiation.
 26. A method as recited in claim 21 wherein said spot ofradiation of said selected wavelength is focused onto said heatedcoating in said area subsequent to said focusing of said infraredradiation onto said area and heating said coating in said area.
 27. Amethod as recited in claim 21 wherein said infrared radiation ismodulated.
 28. A method as recited in claim 27 wherein said spot ofradiation of said selected wavelength and said spot of infraredradiation are the same size and said spots are simultaneously focusedonto said area.
 29. A method as recited in claim 27 wherein said spot ofradiation of said selected wavelength is larger than said spot ofinfrared radiation and said spots are simultaneously focused onto saidarea.